1. Field of the Invention
The present invention pertains to self-propelled lift trucks. Specifically, the present invention pertains to self-propelled lift trucks especially suitable for lifting and lowering of loads onto and from slightly elevated surfaces. Even more specifically, the present invention pertains to self-propelled lift trucks which are suitable for loading and unloading loads onto and from the bed of a truck or trailer and for street or highway transportation on said truck or trailer.
2. Description of the Prior Art
Self-propelled lift trucks have been in existence for many years. The typical lift truck comprises a low heavy frame mounted on wheels with lifting forks extending forwardly therefrom. The forks form part of an assembly which is usually mounted for up and down movement on vertical tracks of a vertical mast of some type. The mast may be tipped or slightly inclined toward the rear of the lift truck to prevent spilling of its load. In addition, heavy counterweights are typically attached to the rear of the lift truck to help counterbalance loads carried on the forks at the forward end thereof since the location of the front wheels at the base of the mast act as a pivot point.
Most self-propelled lift trucks or "forklifts", as they are sometimes called, are typically used for lifting and moving heavy objects from one place to another at the same facility, such as a manufacturing plant. In more recent years, self-propelled lift trucks have been used for loading and unloading heavy loads on and from the beds of trucks or trailers for highway transportation, sometimes without the benefit of a loading dock. Lift trucks of this type usually have some type of mast which allows elevation of the lifting forks, and the loads thereon, at more elevated positions than lift trucks or forklifts of the prior art. This allows loading and unloading from the beds of road vehicles. Examples of such self-propelled lift trucks are shown in U.S. Pat. Nos. 3,826,393 and 4,395,190. To prevent such lift trucks from being overly heavy, they may be designed to straddle the load with their wheels so that the center of gravity, when loaded, is between the front and rear wheels. However, such wide wheel spread may create problems of accessibility to a load and lowering of loads.
Many cargos are loaded with materials from a concentrated source of such materials and transported to points of use where there are no lift trucks for unloading these materials. Unloading of the delivery vehicle in these cases is very difficult and labor intensive. In addition, such unloading is sometimes accomplished by makeshift methods which are dangerous to personnel involved. For this reason, in recent years, some lift trucks have been designed for loading a vehicle and then being loaded on or attached to the vehicle for transportation with the load of materials to its ultimate destination where the lift truck is utilized in unloading the materials. In fact, the self-propelled lift trucks of the aforementioned U.S. Pat. Nos. 3,826,393 and 4,395,190 may have been utilized in this manner. However, in transporting such a lift truck by attaching it to the load carrying vehicle, the lift truck is typically attached to the rear of the vehicle and hangs in a cantilevered fashion from the rear thereof, extending sixty to eighty-four inches rearwardly. Even though the lift trucks designed for this purpose may not be as heavy as some of the lift trucks of the prior art, the fact that the concentration of weight or center of gravity thereof is so far to the rear of the load carrying vehicle's rear axle, may result in accelerated tire wear, instability of the load and less maneuverability of the vehicle. Furthermore, the cantilevered extension of the lift truck to the rear of the vehicle may create safety and traffic hazards and may violate many state traffic laws.
Thus, even though improvements have been made in self-propelled lift trucks which enable the loading and unloading of materials at truck or trailer bed elevations which, in some cases, allows the lift truck to be transported with the load so that it may be used at the point of delivery, further improvements are needed. The self-propelled lift truck disclosed in the aforementioned U.S. patent application Ser. No. 08/094,426 is such an improvement. Such a lift truck provides a forward carriage assembly having a pair of spaced apart ground engageable wheels and a vertical mast on which is mounted a forwardly projecting fork assembly for related upward and downward movement of a load thereon. A rearward carriage assembly, which includes at least one ground engageable wheel and on which may be supported a power source and related controls, is uniquely connected to the forward carriage assembly by at least a pair of spaced apart support members which are pivotally connected at opposite ends to the forward and rearward carriages, respectively. A power device is connected to the support members so that they will move between horizontal positions and vertical positions. Power lift apparatus, by which the fork assembly may be raised and lowered, is also designed to lift the entire lift truck off of the ground for support on the rear of the vehicle on which a load is to be loaded and/or unloaded.
Self-propelled lift trucks which are capable of loading and unloading trucks without a loading dock and which are easily adapted for transport with the load for use in unloading should find great acceptance in many types of industries. Continued improvement in the design of such apparatus will be sought.